1. Field of the Invention
The invention relates to sensors, and more particularly, to fiber optic sensors for detecting the presence or absence of a particular substance.
2. Art Background
Sensors that can detect the presence or absence of a selected substance ("analyte") are widely used in many different fields. For example, environmental sensors are used to detect pollutants and have become increasingly important as environmental standards have become stricter. Medicine is another field where the use of sensors is growing, especially in the area of medical diagnostics. As yet another example, sensors also control various industrial processes. Indeed, sensors perform important functions in many other fields and will undoubtedly find many new applications in the future.
There are many different types of prior art sensors. Fiber optic sensors have increased in popularity due to the ability of a fiber optic cable to inexpensively and accurately carry a signal over long distances.
Prior art fiber optic sensors, however, are not optimal. Prior art fiber optic sensors are either very expensive, inaccurate, or limited in the distance over which they may be used. For example, according to one of the more effective prior art fiber optic sensors, a U shaped fiber is employed for the detection of liquid levels in tanks. See FIG. 1. The U bend is exposed to a liquid, and a light source provides light which is guided to the U bend, where some of it escapes to the environment. Light that does not escape passes through the U bend and is detected by a detector. Because different liquids have different indices of refraction, the amount of leakage through the U bend, and therefore the strength of the signal through the fiber, depends upon the type and/or amount of liquid in contact with the U bend. Thus, continuous monitoring of the variation of the light intensity is used to measure the liquid level in the tank and may also be used to detect the presence of a different liquid in the tank.
However, in practice, prior art U bend sensors do not provide a very strong signal and generally require a relatively high powered, and therefore expensive, light source, especially where the detector must be located far from the U bend portion.
Another scheme to improve the effectiveness of the sensor involves coating the sensor with a substance that dissolves in the presence of a liquid to be detected. When the liquid to be detected is present, it dissolves, thus bringing the sensor directly into contact with its environment and increasing the light loss from the sensor, thus creating a signal. Other schemes include coating a fiber's core with porous soluble materials which change their optical properties based upon the presence/absence of a liquid to be detected.
Again, however, in practice, these mechanisms do not prove particularly effective. In particular, when used in conjunction with present sensor geometries (eg. the U bend described above or a straight line), these schemes generate weak signals per unit area of the fiber and therefore a substantial segment of the fiber must be used as a sensing element and very sensitive, and therefore expensive, detectors are required. Furthermore, because of the fragility of the fibers, it is difficult to package such long sensors or use them in small fluid samples where access to the sample is restricted. Still further, the prior art dissolution mechanisms rely on the presence of a liquid and do not provide for the detection of a solute in a solvent.
The present invention improves the state of the art by, among other things, optimizing the geometrical configuration of the sensor, thus providing a very compact, sensitive mechanism for producing a strong signal. Consequently, the sensors' response to different fluids and chemicals can be accurately monitored from remote locations with off-the-shelf compact light source/detector pairs.